EP2443342A1

EP2443342A1 - Wind turbine foundation for variable water depth

Info

Publication number: EP2443342A1
Application number: EP10789786A
Authority: EP
Inventors: Dagfinn Sveen
Original assignee: Dr Tech Olav Olsen As
Current assignee: Dr Tech Olav Olsen As
Priority date: 2009-06-16
Filing date: 2010-06-16
Publication date: 2012-04-25
Also published as: NO20092311L; KR20120034723A; CN102803720A; WO2010147481A1; NO330475B1; EP2443342A4

Abstract

Wind turbine foundation (1) for variable water depth and method of constructing the wind turbine foundation, comprising a bottom foundation (5), a framework tower (10) provided on the bottom foundation (5) wherein the frame work tower (10) includes at least three parallel tubular legs (12) and a framework system (15) of struts (16) with strut nodes (20) and leg nodes (21) provided between and connected to the legs (12), and a transition structure (25) provided on the upper area of the framework tower, characterized in that the framework tower (10) consists of at least one standardized framework tower element (13, 14) wherein the centre distance between the at least three parallel tubular legs (12) is constant, the diameter of the legs (16) is constant and the strut nodes (20) and leg nodes (21) having standardized shapes.

Description

Wind turbine foundation for variable water depths

The present invention concerns a wind turbine foundation for variable water depth. Particularly the invention is related to a foundation for offshore wind turbines fixed to the sea bottom and extending above the water surface wherein the foundation supports a vertical tubular tower having a wind turbine provided on the top.

Earlier wind turbines were normally located onshore on simple foundations anchored to rock or on casted foundation plates having sufficient weight in order to provide a stabile support for the tower including turbine.

Recently it has been common to locate wind turbines offshore in order to achieve increased access to area and better wind conditions. There are already many examples of wind parks installed in Denmark, Germany, Holland and England.

The first offshore wind parks are located on shallow water with foundation similar to the one onshore. It is inter alia utilized concrete foundations standing stabile by means of weight (gravity) and a single central pile is utilized which is driven sufficiently down in the bottom in order to provide sufficient stability and stiffness for the support of the wind turbine.

These solutions are suited for shallow water depths less than about 20m and for turbines up to about 2-3MW. With larger water depths and heavier turbines with higher towers the prior solutions for small water depths will not be suited since they are generally too flexible which again results in to high structural frequencies. It will be stringent demands to achieve a 1. frequency within an area during the rotation cycle and over 1/3 of the rotation cycle in order to avoid unfortunate frequencies and loads in the structure and the turbine.

Experimentally turbines are installed in water depths up to about 45m. Examples here are the Alpha Ventus project with about 30m water depth and Beatrice with about 45m water depth. On the Alpha Ventus two types of foundations are installed, one is a so called tripod solution and the other is a tubular framework solution corresponding to what is mentioned in the oil field as "jacket". Both foundations are fixed to the bottom with steel piles which are driven down with underwater hammer, corresponding to the method used for offshore platforms. On Beatrice two turbines are installed having jacket foundation which also is fixed with piles to the bottom. With regard to use of steel solutions experience and analysis show that foundation of the type jacket includes large advantages when the water depth increases, i.e. in the area of 30-6Om. In relation to the amount of material the concept generally includes high stiffness and it is simple to achieve the correct frequency also for larger water depths.

The solution also results in low wave forces due to its open structure with slender structure element. The combination of high stiffness and small wave forces is favourable since it results in small wave induced motions in the construction and thereby minimizes the transfer of dynamic forces from waves to tower and turbine. Within the specialist environment it is established a general understanding that the jacket concept technically is a favourable solution for establishing the foundation of wind turbines in the sea. The largest challenge with regard to wind turbines in larger water depths is the costs. In general the costs of establishing the foundation, due to size, complexity and including installation work will be larger than onshore and increasing with increasing water depths.

The offshore industry has experienced that construction and installation of a single platform foundation is extremely expensive due to all engineering, planning, administration and use of offshore installation equipment etc. is related to only one single installation.

The previously mentioned "tripod solution" and "jacket solution" have the disadvantage that they must be adapted or "customized" for the actual location where they are to be installed. I.e. the different structural parts must be formed and dimensioned (designed) according to water depth, environmental forces etc. which results in that the various structural parts will be of varying form, dimension, thickness etc. depending on where they are located in the structure. This result in that such structures in a large degree must be customized to a specific location and hence in a low degree are flexible with regard to mass production. The result is that such structures in a small degree can be mass produced and hence achieve cost reductions when producing a larger number of units.

When manufacturing a larger number of wind turbines for a wind park it is possible to distribute the single costs on many units, further it is possible to in- crease the efficiency of fabrication if the solutions are simple and in a large degree suited for use of automatic working operations. With regard to installation costly equipment such as crane vessel, barges etc. will be used efficient on many installations in addition to that the rates also will be a lot more favourable with regard to longer involvement.

One object of the present invention therefore is to develop a wind turbine foundation which is standardized for variable water depths and with technical favourable solutions and with the lowest possible cost. This means that the best solutions will be a compromise between technology and economics. The most important cost elements are fabrication of the foundation onshore and costs related to transport and installation. These costs will also be related to the possibility to improve the effectivity of fabrication and installation when large scale wind turbines for wind parks are produced.

Another object is that wind turbine foundation can be adapted to different water depths by changing the height of the tower while other main dimensions and structural solutions are kept unchanged. The wind turbine foundation is further in a large degree also standardized independent of water depth such that both detail projecting and administration are simple in addition to that especially developed fabrication equipment and the technical solution can be utilized on as many units as possible.

A further object is that wind turbine foundation shall include good stiffness characteristics and at the same time well suited for effective and in a large degree automized fabrication.

The object of the present invention is achieved by a wind turbine foundation for variable water depth, comprising a bottom foundation, a frame work tower provided on the bottom foundation wherein the frame work tower includes at least three parallel tubular legs and a frame work system of struts with strut nodes and leg nodes provided between and connected to the legs, and a transition structure provided on the upper area of the framework tower, characterized in that the framework tower consists of at least one standardized framework tower element wherein the centre distance between the at least three parallel tubular legs is constant, the diameter of the legs is constant and the strut nodes and leg nodes having standardized shapes. Preferred embodiments of the wind turbine foundation are further defined in the claims 2 to 7.

The object of the present invention is further achieved by a method of constructing a wind turbine foundation for variable water depth, comprising a bottom foundation, a framework tower of struts having struts node and leg nodes and a transition structure, characterized in that the framework tower is provided on the bottom foundation and the transition structure is provided on the upper area of the frame work tower, as the framework tower is provided as in at least a standardized framework tower element wherein the centre distance between the at least three parallel tubular legs is kept constant, the diameter of the struts is kept constant and the strut nodes and the leg nodes are provided as standardized shapes.

A preferred embodiment of the method is further defined in claim 8.

Further features of the invention will become apparent with reference to the attached drawings, wherein: Figure 1 depicts an embodiment of a wind turbine foundation having a gravity based bottom foundation;

Figure 2 depicts a second embodiment of a wind turbine foundation having a bottom foundation including piles;

Figure 3 depicts a node to connect struts of the framework system; Figures 4a and 4b depict a node for connecting struts to the tubular legs of the framework tower; and

Figure 5 depicts a further embodiment of the wind turbine foundation wherein the framework tower consists of two standardized framework tower elements.

With reference to figure 1 and figure 2 a wind turbine foundation 1 is shown supporting a vertical tubular tower with at wind turbine which is provided in its upper tower. The wind turbine foundation 1 comprises a bottom foundation 5, a framework tower 10 provided on the bottom foundation 5 and a transition structure 25 provided on the upper area of the framework tower.

Figure 1 depicts a first embodiment of the bottom foundation 5 in the form of a gravity based foundation 6, preferably of concrete, and possibly with additional ballast in form of gravel or rock. Figure 2 depicts a second embodiment of the bottom foundation 5 where piles 7 are utilized in order to ensure an anchoring in the sea bed. In a third embodiment the bottom foundation 5 can be provided with deep cylindrical steel foundations in each of the corners and which is further driven partly down with the weight (specific gravity) of the installation and in addition the use of vacuum in order to achieve sufficient penetration into the sea bed. It should be mentioned that the choice of bottom foundation 5 will depend on the given bot- torn conditions and other conditions which may have a cost consequence.

Further with reference to figurei and figure 2 the framework tower 10 comprises three parallel tubular legs 12 having equal centre distance between adjoining legs. The three parallel tubular legs 12 have constant diameter from bottom to top. A framework system 15 of strut 16 is provided between and connected to the legs 12. All the struts 16 of the framework system have equal diameter. The struts 16 are provided in a X-system. This result in that a standardized type of node of equal dimension both in the crossing between the struts 16 and for the connection of the struts to the legs 12, strut node 20 and leg node 21, respectively is utilized. In this connection references are made to figure 3, figure 4a and figure 4b. The strut node 21 connecting the struts 16 to the legs 12 is then a so called K-node which constitutes a half of the X-node between the struts 16. Possible variable strength and stiffness degree requiring variable cross section area is achieved by varying the wall thickness of the legs 12 and the struts 16 while all the nods preferably are formed with the same thickness in order to ease the fabrication of these.

With reference to figure 5 a wind turbine foundation 1 is shown wherein the framework tower 10 consists of a first and second standardized framework tower element 13, 14, respectively. The second framework tower element 14 is provided on the top of the first framework tower element 13. Level 11 shows the connection area between the first standardized framework tower element 13 and the second standardized framework tower 14.

Again with reference to figure 1 and figure 2 a transition structure 25 is shown provided on the upper area of the framework tower. It is further assumed that the frame leg distance and the leg diameter are used for variable water depths such that the transition structure 25 between the framework tower and the legs 12, which is the most structural complicated part, can be standardized for one type of wind turbines independent of the water depth. The struts 16 and the strut nodes 20 and the leg nodes 21 will then also have the same dimension for different water levels. The angle of the struts with regard to the horizontal plane shall preferably be 45 degrees, but with variance which is necessary in order to adapt a complete number of strut systems between the bottom level and the top level.

The present invention deals with a wind turbine foundation which is standardized for variable water depth in that it can be constructed in the height by adding new levels of standardized framework tower elements. The result is a very rational and cost efficient mass production. Further, this solution is standardized with regard to turbine size and type, but the water depths can be varied with small effect on the structural solutions. This standardizing combined with mass production results in very high cost efficiency and flexibility in connection with future large scaled wind park developments.

Claims

1. Wind turbine foundation (1 ) for variable water depth, comprising: a bottom foundation (5), a framework tower (10) provided on the bottom foundation (5) wherein the frame work tower (10) includes at least three parallel tubular legs (12) and a framework system (15) of struts (16) with strut nodes (20) and leg nodes (21) provided between and connected to the legs (12), and a transition structure (25) provided on the upper area of the framework tower, characterized in that the framework tower (10) consists of at least one standardized framework tower element (13, 14) wherein the centre distance between the at least three parallel tubular legs (12) is constant, the diameter of the legs (16) is constant and the strut nodes (20) and leg nodes (21) having standardized shapes.

2. Wind turbine foundation (1) according to claim 1 for variable water depth, characterized in that the framework tower (10) comprises a first and a second standardised framework tower element (13, 14), respectively wherein the second framework tower element is provided on top of the first framework tower element.

3. Wind turbine foundation (1) according to claim 1 or 2, characterized i n that the tubular legs (12) are for the specific areas adapted with regard to strength by varying wall thickness.

4. Wind turbine foundation (1) according to any one of the previous claims, characterized in that the struts (16) are for the specific areas adapted with regard to strength by varying wall thickness.

5. Wind turbine foundation (1) according to any one of the previous claims, characterized in that the strut nodes (20) and the leg nodes (21) having similar and standardized wall thickness.

6. Wind turbine foundation (1) according to anyone of the previous claims, characterized i n that the strut nodes (20) having a X-node shape and the leg nodes (20) having a K-node shape as the K-node shape constitutes one half of the X-node shape.

7. Method of constructing a wind turbine foundation (1) for variable water depth, comprising a bottom foundation (5), a framework tower (15) of struts (16) having struts node (20) and leg nodes (21) and a transition structure (25), cha racterized in that the framework tower (10) is provided on the bottom foundation (5) and the transition structure (25) is provided on the upper area of the framework tower, as the framework tower (10) is provided as in at least a standardized framework tower element (13, 14) wherein the centre distance between the at least three parallel tubular legs 12 is kept constant, the diameter of the struts (16) is kept constant and the strut nodes (20) and the leg nodes (21) are provided as standardized shapes.

8. Method according to claim 7, characterized i n that the framework tower (10) is comprised of a first and a second standardized framework tower element (13, 14), respectively wherein the second framework tower element (14) is provided on top of the first framework tower element (13).